The molecular properties of purified rat liver HMG-CoA reductase (HMGR) have been investigated. A molecular weight of 320,000 was obtained for holoenzyme by sedimentation equilibrium and high performance liquid chromatography (HPLC). The purified enzyme migrated as two bands on analytical slab-SDS-gel electro-proresis. The monomer molecular weights by sedimentation equilibrium for upper and lower bands were 54,000 and 52,000 respectively. Purified HMGR can be separated into active and inactive forms by sucrose density gradient centrifugation. On slab-SDS-gel electrophoresis the upper (54,000) and lower (52,000) bands were derived from inactive and active forms of HMGR. Under different physiological situations the ratio of active and inactive forms of HMGR shifted drmatically. The interconversion of two species of HMGR may represent an additional mechanism for the in vivo regulation of cellular cholesterol biosynthesis. The cytosolic reductase kinase (RK) and mevalonate kinase (MK) have been separated by three different techniques. Fractions containing only RK reversibly inactivated HMGR. Fractions containing only MK or mixture of MK and RK revealed artifactual RK activity in the absence of EDTA or mevalonate; however addition of EDTA or mevalonate before HMGR assay eliminated any apparent decline in HMGR activity.